Thermal transfer systems have been developed to obtain prints from pictures, which have been generated from a camera or scanning device. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are converted into electrical signals. These signals are operated on to produce cyan, magenta and yellow electrical signals. The signals are transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Dye receiving elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and, optionally, additional layers. The dye image-receiving layer conventionally comprises a polymeric material chosen from a wide assortment of compositions for its compatibility and receptivity for the dyes to be transferred from the dye donor element. Dye must migrate rapidly in the layer during the dye transfer step and become immobile and stable in the viewing environment. Care must be taken to provide a receiving layer which does not stick to the hot donor as the dye moves from the surface of the receiving layer and into the bulk of the receiver. An overcoat layer can be used to improve the performance of the receiver by specifically addressing these latter problems. An additional step, referred to as fusing, may be used to drive the dye deeper into the receiver.
The receiving layer must act as a medium for dye diffusion at elevated temperatures, yet the transferred image dye must not be allowed to migrate from the final print. Retransfer is potentially observed when another surface comes into contact with a final print. Such surfaces may include paper, plastics, binders, backside of (stacked) prints, and some album materials. Polycarbonates and polyesters have both been used in image-receiving layers. The term “polycarbonate” as used herein means a polyester of carbonic acid and a diol or diphenol. For example, polycarbonates have been found to be desirable image-receiving layer polymers because of their effective dye compatibility and receptivity. As set forth in U.S. Pat. No. 4,695,286, incorporated herein by reference, bisphenol-A polycarbonates of number average molecular weights of at least about 25,000 have been found to be especially desirable in that they also minimize surface deformation which may occur during thermal printing. These polycarbonates, however, do not always achieve dye transfer densities as high as may be desired, and their stability to light fading may be inadequate. U.S. Pat. No. 4,927,803, incorporated herein by reference, discloses that modified bisphenol-A polycarbonates obtained by co-polymerizing bisphenol-A units with linear aliphatic diols may provide increased stability to light fading compared to unmodified polycarbonates. Such modified polycarbonates, however, are relatively expensive to manufacture compared to the readily available bisphenol-A polycarbonates, and they are generally made in solution from hazardous materials (e.g. phosgene and chloroformates) and isolated by precipitation into another solvent. The recovery and disposal of solvents coupled with the dangers of handling phosgene make the preparation of specialty polycarbonates a high cost operation.
Polyesters, on the other hand, can be readily synthesized and processed by melt condensation using no solvents and relatively innocuous chemical starting materials. Polyesters formed from aromatic diesters (such as disclosed in U.S. Pat. No. 4,897,377, incorporated herein by reference,) generally have good dye up-take properties when used for thermal dye transfer; however, they exhibit severe fade when the dye images are subjected to high intensity daylight illumination. Polyesters formed from alicyclic diesters are disclosed in U.S. Pat. No. 5,387,571 of Daly, incorporated herein by reference, the disclosure of which is incorporated by reference. These alicyclic polyesters also generally have good dye up-take properties, but their manufacture requires the use of specialty monomers which add to the cost of the receiver element. Polyesters formed from aliphatic diesters generally have relatively low glass transition temperatures, which frequently results in receiver-to-donor sticking at temperatures commonly used for thermal dye transfer. When the donor and receiver are pulled apart after imaging, one or the other fails and tears and the resulting images are unacceptable.
U.S. Pat. No. 5,302,574 to Lawrence, incorporated herein by reference, et al. discloses a dye-receiving element for thermal dye transfer comprising a support having on one side thereof a dye image-receiving layer, wherein the dye image-receiving layer comprises a miscible blend of an unmodified bisphenol-A polycarbonate having a number molecular weight of at least about 25,000 and a polyester comprising recurring dibasic acid derived units and diol derived units, at least 50 mole % of the dibasic acid derived units comprising dicarboxylic acid derived units containing an alicyclic ring within two carbon atoms of each carboxyl group of the corresponding dicarboxylic acid, and at least 30 mole % of the diol derived units containing an aromatic ring not immediately adjacent to each hydroxyl group of the corresponding diol or an alicyclic ring. The alicyclic polyesters were found to be compatible with high molecular weight polycarbonates.
U.S. Pat. No. 4,908,345 to Egashira et al., incorporated herein by reference, discloses a dye receiving layer comprising a phenyl group (e.g. bisphenol A) modified polyester resin synthesized by the use of a polyol having a phenyl group as the polyol compound. U.S. Pat. No. 5,112,799, also to Egashira et al., incorporated herein by reference, discloses a dye-receiving layer formed primarily of a polyester resin having a branched structure.
Polymers may be blended for use in the dye-receiving layer in order to obtain the advantages of the individual polymers and optimize the combined effects. For example, relatively inexpensive unmodified bisphenol-A polycarbonates of the type described in U.S. Pat. No. 4,695,286 may be blended with the modified polycarbonates of the type described in U.S. Pat. No. 4,927,803 in order to obtain a receiving layer of intermediate cost having both improved resistance to surface deformation which may occur during thermal printing and to light fading which may occur after printing.
It is always desirable to improve image recording elements with an image-receiving layer in terms of providing excellent image properties and economic manufacture. It would be especially desirable to provide a receiver element for thermal dye transfer processes with an image receiving layer having excellent dye uptake and image dye stability, retransfer resistance, and which can be effectively printed in a thermal printer.
A vast majority of dye receiver layers (DRL) are coated from solvent based compositions, for example, U.S. Pat. Nos. 5,411,931; 6,096,685; 6,291,396; and references therein, all incorporated herein by reference. In order to improve adhesion of the dye receiving layer to the support of the imaging element, U.S. Pat. Nos. 4,774,224; 4,814,321 and 4,748,150, all incorporated herein by reference, disclose dye-receiving elements for thermal dye transfer comprising polyethylene coated supports having thereon a subbing layer of a vinylidene chloride copolymer coated out of a solvent and a polymeric dye image-receiving layer, also coated out of a solvent. While the use of such vinylidene chloride copolymer subbing layers improves the adhesion of the dye image-receiving layer to polyethylene coated supports, it has been found, as stated in U.S. Pat. No. 4,965,238 that adhesion to other polyolefins such as polypropylene is not as good. Even in the case of polyethylene, in some instances where the use of vinylidene chloride copolymers gives apparently acceptable initial adhesion, adhesion after thermal transfer of a dye image is poor.
U.S. Pat. Nos. 4,965,238 and 4,965,239, both incorporated herein by reference, disclose use of subbing layers which are solvent coated and the polymer used for the subbing layer has an inorganic backbone of an oxide of zirconium or titanium. An alcohol based subbing layer has been disclosed in U.S. Pat. No. 5,384,304, incorporated herein by reference, wherein a mixture of an aminofunctional organo-oxysilane and a hydrophobic organo-oxysilane is coated over the support, and further overcoated with a dye receiving layer from a solvent based composition. U.S. Pat. No. 5,858,916, incorporated herein by reference, discloses a similar subbing layer comprising a mixture of an aminofunctional organo-oxysilane and a hydrophobic organo-oxysilane, but further comprising a salt for ionic conductivity, for reducing charge generation during transport through a thermal printer. U.S. Pat. No. 6,881,704, incorporated herein by reference, disclosed solvent based intermediate layers comprising a resin and acicular inorganic particles, overcoated with solvent based DRL, for alleged improvement in cracking.
Thermally extruded dye receiver layers (DRL) have recently been introduced for thermal receiver media, as disclosed in U.S. Pat. Nos. 6,893,592 and 6,897,183, incorporated herein by reference. These patents teach a polyester-based dye receiving layer extruded onto a support that typically comprises a paper core laminated with a microvoided polyolefin sheet, usually biaxially oriented polypropylene (BOPP), on the face side and a non-microvoided polyolefin sheet, also preferably a biaxially oriented polypropylene, on the backside. As compared to a solvent coated dye receiving layer, the extruded dye receiving layer has significantly less adhesion to the support. The polyester based dye receiving layer alone, in which the polypropylene which is amorphous, has very little adhesion to the substrate which is typically semi-crystalline and requires an adhesion promoting tie-layer, as described in U.S. Pat. Nos. 6,893,592, co1.7, lines 56-60, incorporated herein by reference.
U.S. Pat. Nos. 6,893,592 and 6,897,183, incorporated herein by reference, disclose polyolefin based tie-layers that are co-extruded with the dye receiving layer onto the substrate. Although adhesion is adequate for kiosk printers, it has recently been observed that there is scope for improvement in the adhesion of the dye receiving layer with such co-extruded tie-layers for printing in new generation of thermal home printers that utilize borderless (full bleed) printing, providing edge-to-edge image coverage. In case of inadequate adhesion, printing in these printers can run the risk of print delamination, resulting in unsatisfactory prints.
The polyolefin based tie-layers described in the aforementioned two patents also serve to provide antistatic characteristics. The preferred antistatic materials comprise polyether based polymers. These polymers typically provide humidity sensitive ionic conductivity. Numerous such polymers are disclosed in U.S. Pat. Nos. 6,197,486; 6,207,361; 6,838,165; 6,872,501 and references therein, all incorporated herein by reference.
Antistatic materials for imaging elements that comprise humidity-independent electronic conductors are also well known in the art. These electronic conductors can be metal-containing particles, conjugated polymers, various forms of carbon and the like. Application of such materials in imaging has been described in detail in the patent art; vide for example U.S. Pat. Nos. 5,368,995; 5,719,016; 6,124,083; 6,429,248; 6,566,033 and references therein, all incorporated herein by reference. Although some of these patents may suggest the use of these antistatic layers in imaging elements including thermal receivers, there is no description of any thermal media wherein a dye receiving layer is thermally extruded over such antistatic layers.
U.S. Pat. No. 5,719,016 teaches of an antistatic layer comprising acicular tin oxide dispersed in a binder incorporated in imaging elements. Although the imaging elements include thermal receivers, there is no description of any thermal media wherein a dye receiving layer is thermally extruded over the antistat layer. U.S. Pat. No. 5,719,016 does not disclose a binder with strong adhesion to biaxially oriented polypropylene and thermally extruded polyester. In fact, the binders used in Examples 3-5 (i.e., terpolymer of acrylonitrile, vinylidene chloride and acrylic acid and polyester ionomer) have little adhesion to biaxially oriented polypropylene and will not be useful in extruded thermal receiving elements.
U.S. Pat. No. 5,718,995 teaches of an antistatic layer comprising a conductive agent and polyurethane with an ultimate elongation to break of at least 350 percent, coated over energetically treated polyester surface. The aforesaid antistatic layer was further overcoated with a transparent magnetic layer, which is coated from a solvent, as per Research Disclosure, Item 34,390. U.S. Pat. No. 5,718,995, however, does not mention of any thermally processable layer extruded over the antistat layer.
U.S. Pat. No. 5,646,090 discloses thermal transfer image receiving sheet comprising a substrate, an intermediate layer containing polyurethane of Tg≧40° C., and a dye receiving layer. However, in this patent, the intermediate layer and the dye receiving layer are both coated out of solvent mixtures, and no teaching is made of any thermally extruded layer.